Increasing populations of feral or stray domestic dogs and cats has been a growing problem in North America and the rest of the world. For example, an estimated 40% of domestic cats (Felis catus) in the United States are classified as feral or stray.
Concerns about impacts on wildlife, transmission of infectious diseases, and the welfare of the cats and dogs themselves have led to various strategies to reduce the number of feral cats and dogs. Locally, and throughout the world, extermination has been the dominant method used in the attempt to control free-ranging feral cats and dogs.
Surgical sterilisation of feral cats and dogs by veterinarians followed by release back into the colony has been increasingly utilised as a humane tool to lower feral cat and dog populations in the last 2 decades. Despite the success of large-scale surgical sterilisation, such programs are not financially or logistically feasible in many locations.
During the last decade, interest has increased in applying immunocontraception (IC) as a reliable method to lower population of pest species. IC can be a humane means of reducing fertility in domestic, feral and wild mammals (Oogjes, 1997), and several potential IC targets exist. For example, a vaccine that used gonadotrophin-releasing hormone (GnRH) as antigen, depressed ovarian activity in horses for one breeding season (Bradley et al., 1999). The difficulty with GnRH directed vaccines is that there is a potential for endocrine dysfunction (Muller et al., 1997).
Zona pellucida (ZP), a noncellular glycoprotein coat surrounding the mammalian egg, regulates sperm-egg interaction during fertilisation (Sacco and Yurewicz, 1989). This structure is an ideal candidate for a contraceptive target, since altering its structure can prevent pregnancy. ZP immunisation has been effective in lowering fertilization rates of many mammals (Willis et al., 1994; Kirkpatrick et al., 1996; Brown et al., 1997a,b; Harris et al., 2000). Two independent reports have indicated that pig zona pellucida (pZP) is an effective immunocontraceptive (although requires multiple boosters) in domestic cats (Ivanova et al., 1995; Bradley et al., 1999). Porcine zona pellucida has also been used in liposome-based immunocontraceptive vaccines for reducing fertility of certain mammals by 90–100% with a multi-year efficacy (Brown et al., 2001). However, use of pZP in such a liposome-based vaccine as a single administration vaccine for cats is ineffective in cats (Gorman et al., 2002).